Just eyeball the length of the resistance wire. Remember that whatever resistance you make each one (and do make them both the same length) the overall resistance will be halved. So if you make two 2.0 Ohm coils your overall Dual Coil resistance will be 1.0 Ohm. So if you normally make lets say a single coil at 1.3-1.5 Ohms you know how long your resistance wire is. When you make a dual coil make the two resistance wires longer than the single you usually make.